A donor in a "window" period - when antibodies do not show up in the blood - or a failure in the screening process were the most likely causes of contamination of the plasma pool with traces of the hepatitis C virus.
The National Blood Authority said that recalling the products was precautionary and assured patients that the four batches of factor VIII and albumin would pose no risk to them. The blood products would have gone through viral inactivation processes to make them safe even if they did contain a virus, the NBA said.
Factor VIII is given to haemophiliacs and albumin is used to treat people with burns and shock. The batches, sent out in June and July, contained about 2,000 bottles of factor VIII and 11,500 of albumin.
The problem was revealed by a new extra-sensitive test known as the PCR (polymerase chain reaction), required under European rules for the manufacture of certain types of blood product. Minute traces of hepatitis C were found in the plasma pool from which the products were being made by Bio Products Laboratory (BPL), which is part of the NBA.
The test was carried out on a sample of the plasma that was to be used to make immunoglobulin - an immune system booster used to fight hepatitis and one of the vaccinations commonly given to travellers. The plasma pool had already been used to manufacture factor VIII and albumin.
A spokeswoman for the NBA said: "We consulted the Department of Health and felt in the public interest that this was the right thing to do. If we find any trace of a virus then we don't use the product."
She stressed that there was no chance of the virus posing a health risk. Factor VIII and albumin were processed to knock out any viruses they might contain before they were given to patients, she said. "Patients who have used the products need have no concern about their safety."
She said that the virus could have made its way into the plasma pool if one of the donors had been in a "window" period or if there had been a fault in one of the tests. She said that a full investigation would be carried out to find out how this could have happened.
Gabrielle Page, spokeswo-man for the hepatitis C support group, said that the "fragmentation" of the blood service made it easier for mistakes to happen. "Whereas it used to be a body, non-profit making and existing for itself, now that t has to sell off its products and has to become diversified it has changed."
BPL was yesterday contacting its customers asking them to return the blood products, which would be destroyed. The plasma pool would not be used to make any more products.
Hepatitis C is a "silent" disease which may not produce symptoms for 20 years. It was discovered only in 1989 and, without treatment, 25 to 50 per cent of patients develop scarring of the liver and a proportion of those will have liver failure and some will develop liver cancer.